Drug-induced adverse effects in human and animals are known and have been largely associated with the malfunctioning of major organs such as heart, liver, and kidney. The potential for adverse drug effects adds a level of complexity to safe therapeutic application. Along with hepatotoxicity, assessment of cardiotoxicity is a significant component of drug discovery and the drug development process. Drug-induced cardiotoxicity is an adverse event associated with certain drugs including some chemotherapeutic agents used to treat hematologic and solid malignancies, and often leads to morbidity and high mortality. Numerous biochemical, cellular and tissue studies in the past decades have suggested that drug-induced cardiac abnormality is associated with the changes in the activity of, for example, ion channels, myocyte structure, extracellular matrix (ECM) structure, and neurohumoral system. Drug cardiotoxicity can also results in protein abnormalities related to Ca2+ and abnormalities of the signaling system. Among these factors, the change in ion channel activity has been recognized as a major cause of drug-induced cardiotoxicity. Numerous overlapping ionic currents contribute to regulate the morphology and duration of ventricular action potential duration. Depolarization of the ventricles is initiated by the rapid entry of Na+ through selective sodium channels. This is followed by a rapid repolarization through transiently activating and inactivating outward potassium channels, and subsequently by a plateau phase, mainly determined by the entry of calcium ions through L-type calcium channels. During repolarization the negative transmembrane potential is recovered by the inactivation of calcium channels and the increase in net outward potassium currents carried mainly by the slow and rapid components of the delayed rectifier potassium channels. Inwardly-rectifying potassium channels also contribute to the repolarization. The regulatory factors including Na+/K+-pump restore intracellular ion concentrations to the original state.
Assessment of drug cardiotoxicity is significant for the safe development of novel pharmaceuticals. Assessing a compound's risk for prolongation of the surface electrocardiographic QT interval and hence risk for life-threatening arrhythmias is an FDA (US Food and Drug Administration) mandate before approval of nearly all new pharmaceuticals in the US. The QT interval is a measure of the time between the start of the Q wave and the end of the T wave in the heart's electrical cycle. In general, the QT interval represents electrical depolarization and repolarization of the left and right ventricles. A lengthened QT interval is a biomarker for ventricular tachyarrhythmias, such as torsades de pointes (TdP), and a risk factor for sudden death. QT prolongation has most commonly been associated with loss of current through hERG (human ether-a-go-go related gene) potassium ion channels due to a direct block of the ion channel by drugs or occasionally by inhibition of the plasma membrane expression of the channel protein. In the majority of cases, drugs that prolong the QT interval preferentially inhibit the rapid component of the delayed rectifier potassium current channel (I(Kr)), or hERG, the gene that encodes for the alpha-subunit of IKr channels.
A widely recognized in vivo drug cardiotoxicity assessment method is the electrocardiogram (ECG) test. The surface ECG provides information on the electrical events including atrial/ventricular depolarization and ventricular repolarization within the heart. In ECG, the QT interval indicates ventricular depolarization (i.e., a decrease in the electrical potential across a membrane) and repolarization (i.e., recovery of the resting potential). The ECG represents the duration of the ventricular action potential and includes the QT interval, which interval reflects activation time of both ventricles. Although most drug-induced QT prolongation is associated with the inhibition of hERG, the opposing correlate that inhibition of the hERG channel causes a long QT interval has not been conclusively proven. In addition, cardiotoxicity can also be generated by changes in ion pump activities and by cardiomyocyte cell death. Early identification of adverse drug-induced risk can be beneficial in assessing multiple ion channels at a molecular level, cell proliferation, and cell death. Cardiotoxicity testing has become a central component of drug development.
There are several other techniques used for in vitro evaluation of drug-induced cardiotoxicity, including the patch clamp technique using hERG transfected cells or isolated cardiomyocytes, Rb+ efflux assay, microelectrode assay using Purkinje fibers or guinea pig papillary muscle, and bioimpedance based cardiomyocyte profiling. Among them, the patch clamp technique is probably the most widely used tool for screening of drug-induced cardiotoxicity that allows monitoring the effect of only a single drug on a single target ion channel at a time. Although this technique provides high accuracy, it does not allow for contemporaneous observation of multiple ion channel activities, and does not allow for systematic presentation of drug cardiotoxicity including cell death and alterations in cell signaling. Most of the other available technologies are associated with the averaged measurement of a population of cells. A label-free whole cell assay method that can integrate single cell biology with population behavior of cardiovascular cells would be highly advantageous.